1. Field of the Invention
This invention relates to a self-supporting bladder such as can be used to retrofit an existing single wall storage tank into a double wall storage tank
2. Description of Related Art
Commercial and industrial liquids of all types are stored in storage tanks. One of the most notable such types of storage is for motor vehicle fuel. For reasons of space, many of these tanks are placed underground to be able to supply filling stations and other places where large amounts of liquids are to be stored and distributed. Underground fuel tanks are generally cylindrical in shape and usually have a capacity in the range of 500 to 20,000 gallons or more. Such tanks are generally made of either metal (usually steel) or a fiber reinforced resinous material to resist the often corrosive nature of the materials stored in them.
Storage tanks also are used in a variety of other situations for the storage of liquids. Tanks can be used in industrial settings for the storage of liquids to be used in manufacturing, can be used as storage for end products prior to shipping, or can be used as part of a manufacturing process. Storage tanks are quite ubiquitous and can store all manner of liquids. They are commonly used for the storage of fuels, water, foods, and valuable chemicals, but also can be used to store waste or raw materials. While most of these tanks are in a generally standard size range, they can range from small sizes of less than 100 gallons, to massive tanks the size of small buildings.
Because the liquids stored in such tanks are often hazardous (gasoline for use as a motor fuel being one of the most common), and thus can cause severe environmental damage and greatly impact the lives of people living, working, and recreating in nearby areas, careful attention to the potential for leaks from such tanks must be exercised. In some cases, even small leakage from such an underground tank can have profound effects as the chemical can serve to poison a local water supply, or lead to a situation where nearby soil or other materials become directly hazardous to life. The fact that many of these storage tanks are in areas of higher population density simply exaggerates the problems.
Due to these potential problems from leaks, safer storage tanks have been designed with a double wall, such that a breach in the integrity of either of the inner or outer wall alone will not allow a leak of the liquid contained in the tank outside of the tank. The use of such double-walled tanks (or equivalents thereof, wherein some sort of secondary containment is provided for an otherwise single-walled tank) is increasingly being mandated by government regulation, particularly when such tanks are placed underground or in places where a leak could potentially contaminate soil, air, or water.
While new tanks can be built to more stringent safety standards, because a large number of tanks have already been placed prior to the rules being imposed and the operation for removing and replacing them can be economically unviable, it is often the case that tanks need to be retrofitted in place to comply with more stringent safety regulations or simply taken out of service, which results in a major waste of resources. In one alternative, a tank structure that provides added safety from the hazards of leaking storage tanks comprises the retrofit of a liner which is installed in a single wall tank that has been in use and is already in the ground or in position for use. Certain of these liners can be installed without removing the tank from its original position. Such a lining can be significantly more economical to install as compared with removal and replacement of the single-walled tank with a new double-walled tank.
In one arrangement, a rigid tank liner is used to line the existing tank with a second (and, in some cases, third) wall. A method of retrofitting tanks has been described in U.S. Pat. No. 5,904,265, the disclosure of which is entirely incorporated herein by reference. This type of method includes providing a double-layered liner comprising a multi-layered fabric having an interstitial space between two generally parallel layers of fabric, the layers being supported at a distance from one another by generally perpendicular fabric pylons, all of which is reinforced and hardened by a resin polymerization.
Resin tank liners generally require the existing tank to be completely emptied and then thoroughly cleaned before it can be retrofitted. If the existing tank is not properly cleaned prior to the liner installation, the resin liner would not properly adhere to the existing tank and this will result in the resin liner collapsing under the strain of ordinary operation. This cleaning operation, and the installation of the liner itself, all require entry into the tank by personnel to perform the cleaning and installation operations.
The environment inside a storage tank can be incredibly hazardous. For one, the tank may still include gaseous fumes from the material it was previously filled with which can be toxic and result in asphyxiation or other respiratory problems for personnel. Thus, in such cleaning and related operations, personnel may be required to wear heavy, bulky, and cumbersome protective equipment. Even beyond that, such fumes, for example in a tank which had stored motor fuel, can be highly explosive. This can require the cleaning operation to use specialized tools to try and prevent explosion of the tank during the cleaning and installation activities. It also can require personal safety gear to be redesigned to operate in such an environment to insure that a piece of gear designed to prevent inhalation of fumes, does not serve to ignite them. These issues result in an increased cost in performing the retrofit, as well as often requiring the retrofit to take a longer time, resulting in a longer downtime for the tank.
In order to avoid the problems with a liner, in some situations a flexible bladder is used to allow for installation in situ. In this arrangement, the bladder is placed into the empty tank in a generally collapsed state and is then expanded to fill the internal area inside the tank. The liquid to be placed into the tank is then inserted into the bladder. As the bladder conforms to the internal space of the original outer tank, a double wall tank is created with the bladder as the inner wall and the original outer tank as the outer wall.
One of the advantages of using bladders as opposed to more complex methods is that bladder installation generally does not require entry of personnel into a tank and the internal surfaces of the tank need not be specially prepared for the installation of the bladder. Because the bladder is effectively constructed outside of the tank, it can be inserted into the tank in a collapsed state, and then expanded in place. Further, any small amount of residue will generally not effect safe operation of the resultant double wall tank.
Because the bladder is flexible, it is necessary to hold the bladder in place against the inner surface of the exterior rigid tank in order to give it shape and so as to provide for it to be supported within the tank. Should the bladder be allowed to freely move internal to the tank, the bladder could easily become torn or interfere with operations in the tank as the liquid internal to it forced the bladder to conform to the internal shape of the tank.
In traditional arrangements, the bladder is held in place at certain easily accessible rigid attachment points (such as by attaching it to the tank at an area around an access point), and by then pulling a vacuum between the outer wall of the bladder and the internal wall of the tank. The vacuum serves both to keep the bladder in place in close proximity to the tank wall and to provide a first indication of a leak. Vacuum systems also are desirable because they are relatively simple, and do not require tank entry by personnel to install.
The vacuum system assists in detecting a leak because should the bladder or tank develop a leak, the vacuum will generally not be formed and the failure of the vacuum can be detected. As such failure is indicative that a leak has occurred, the tank requires maintenance to prevent leakage of the fluid therein and such maintenance can be quickly scheduled and the liquid can be removed to a different storage tank before structural failure of the second wall occurs. The problem with the system, however, is that it presumes that the outer tank remains structurally sound in order to maintain structural integrity and when the bladder or outer wall fails, the inability to draw the vacuum may result in further problems.
Because the internal bladder is not structurally self-supporting, should the vacuum fail, the bladder will cease to be held against the inner surface of the existing tank and will instead generally conform to the liquid surface (as the liquid is constrained by the bladder and tank). As the liquid level drops, this can result in the bladder dangling from the rigid attachment at the access point, eventually tearing as the dead weight of the bladder (and the liquid within it) exceeds the attachment point strength, or the strength of the material forming the bladder, and falling into the tank bottom where it can stifle or otherwise interfere with internal pumps. Thus, should a bladder system fail, the tank generally has to be taken completely out of service and the bladder be replaced. Further, the act of getting the tank emptied to perform such service may cause further damage to the bladder and result in further complications for the resultant cleanup.
A further problem with bladder systems is that they are generally reliant on the existing outer tank to form the second wall of the double wall system. Thus, should the outer tank develop a leak, the system no longer will comprise a double wall system and will often have to be taken out of service until the existing tank can be fixed. This is often an expensive and time consuming process, if it is possible at all. Thus, should the existing tank develop a leak, the system may be determined to completely fail and have to be replaced as the system is dependent on the existing tank for support of the bladder and as the second wall of the double walled system.